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Mr Corbyn had insisted ahead of the conference that his party was now the “mainstream” in British politics and should be preparing to form a Government.

But, despite his popularity with young people, the poll showed just 26 per cent of voters aged 65 to 74 would back his party.

By contrast, 63 per cent of voters aged 18-24 supported Labour, a rise on the 53 per cent who backed Mr Corbyn at the ballot box.

Labour officials are said to be extremely worried that Mr Corbyn’s left-wing rhetoric could be alienating a large proportion of the population.

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Jeremy Corbyn is trailing in the polls

Despite trying to lure the older vote with policies like the triple lock on pensions and winter fuel payments, many of those surveyed showed they still did not trust his party on the economy.

Adam Drummond, from Opinium, told the Observer: “Their best chance of making any gains with older voters lies in making the party a safe choice for those who would like to kick out the incumbent government but are more worried about the opposition.”

Mr Corbyn’s net personal ratings have also taken a tumble, falling from - five per cent to -10, still a point ahead of Mrs May.

The poll, based on 2,004 online interviews, showed Vince Cable’s Lib Dems were stuck on just six per cent, with Ukip and the SNP on just four per cent each.

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Jeremy Corbyn is being urged to change his Brexit policy

Despite the warning signs, Mr Corbyn said in an article for the Observer that the party's unexpected gains in the June general election showed his party is now a "government in waiting".

He wrote: ”We have changed the political centre of gravity. We are now the political mainstream and have the chance to transform our country.

"To do that we must use our new strength inside and outside parliament to challenge the Conservatives at every step - and prepare to form a government to change Britain when the next election is called.”

The leader held a huge Glastonbury-style rally to cheering supporters in Brighton yesterday in which he pledged to deliver “social justice”.

But Mr Corbyn was facing a challenge of his own on Sunday, with 30 senior Labour figures calling on him to keep the UK in the single market after Brexit in a bid to put some clear water between their party and the Tories.

Shadow Brexit secretary Sir Keir Starmer has previously said that under a Labour government Britain would remain in the single market and the customs union for a transitional period of two to four years after Brexit.

However the signatories to the letter published in The Observer, including former shadow cabinet members Chuka Umunna and Heidi Alexander, as well as long-standing Corbyn ally Clive Lewis, said the party should go further to protect jobs and workers' rights.

The letter, published in the Observer, said Labour needed to present an alternative to the Tories' "destructive Brexit".

It said: ”The supposed benefits of a clean break with the EU are a fantasy.

“The economic impact of leaving the single market would hit the most vulnerable in our society hardest.”